Portable blade sharpening tools have been used for many years. An example of such tools in the prior art is Hermann, U.S. Pat. No. 2,897,640, which discloses a blade sharpener used for sharpening a blade's elongated, beveled cutting edge. The Hermann blade sharpener uses two handles to press the grinding stone against the cutting edge of the blade, one handle attached to a backing plate which is pressed against the back (non-sharp) edge of the blade, and a second handle attached to a small frame that has the grinding stone mounted to it. The grinding stone is pressed against the beveled cutting edge of the blade as it is sharpened. The Hermann apparatus necessarily requires two hands in order to operate its function. A second example of such tools in the prior art is Holland, U.S. Pat. No. 2,993,312, which discloses a blade sharpener which is attached to an electric drill. In Holland, a cylindrical sharpening stone is rotated against the cutting edge of a blade, and can be used in some cases while the blade is still attached to its machine. A tapered guide is provided which assists in holding the sharpening stone against the beveled cutting edge of the blade at the correct angle. The tapered guide rotates along with the sharpening stone in order to reduce friction at the apparatus is moved along the edge of the blade.
Other portable blade sharpening tools in the prior art have been specifically designed to sharpen blades of lawn mowers, both reel-type and rotary-type mowers. Most of such tools have been designed so as to be used while the blades are still attached to their respective mowers. One example of a rotary mower blade sharpening tool is Kolling, U.S. Pat. No. 3,122,865, which discloses a rotary grinding stone and guide that is attached to an electric drill. Both the grinding stone and guide rotate, and both are beveled so as to hold the blade being sharpened at the correct angle. The two beveled surfaces (of the stone and the guide) define an acute angle (60 degrees in the preferred embodiment) which renews the sharp edges of a blade with little or no tendency to form burrs on the blade. The user of the apparatus simply presses it down firmly against the cutting edge of the blade in order to sharpen it.
Another example of a rotary mower blade sharpening tool is Ralston, U.S. Pat. No. 3,139,710, which uses an electric motor or drill to rotate a grinding wheel which sharpens the blade. The apparatus includes a frame that is pressed against the back side of the blade to position the blade at the correct angle to the grinding wheel. The frame has a circular guide that the rear edge (the non-sharp edge) of the blade is pulled up against. The Ralston grinding wheel is not tapered, but rather is completely cylindrical. The side wall of the cylindrical grinding wheel is pressed against the cutting edge of the blade as the blade is being sharpened. The position of the circular guide in the frame can be adjusted to adjust for blades of varying width.
Another example of a rotary mower blade sharpening tool is Keating, U.S. Pat. No. 3,800,480, which has a guide plate along the side of the motor housing that is pressed against the back surface of the blade. The motor output shaft is coupled to a pinion, which is, in turn, coupled to a spur gear. The spur gear is coupled to a grinding wheel, which is pressed against the leading edge of the blade (the cutting edge) at an acute angle. The angle of the grinding wheel against the blade cutting edge is determined by the angle of the coupling between the pinion and the spur gear. This system is relatively intricate from a mechanical standpoint, since the angle of cutting cannot be changed easily.
Some of the devices of the prior art are somewhat unwieldy in that a relatively large portion of the device must be placed over the opposite side of the blade to be sharpened, and there is only limited space available in the interior regions of the lawn mower that is having its blade sharpened. Holland, U.S. Pat. No. 2,993,312, discloses a cylindrical grinding wheel having its entire bulk within the interior region of the mower. Keating, U.S. Pat. No. 3,800,480, discloses a spur gear with grinding wheel attached that must fit into the interior region of the mower. Ralston, U.S. Pat. No. 3,139,710, discloses a frame with a circular guide that must fit into the interior region of the mower. Ralston's FIG. 1 depicts enough clearance to enable the apparatus to fit within the required space, but it is obvious that the smaller the size of the portion of the apparatus that fits into the interior region of the mower, the more likely that the apparatus will be easily usable.